Engine air-fuel ratio may be maintained at a desired level (e.g., stoichiometric) in order to provide desired catalyst performance and reduced emissions. Typical feedback air-fuel ratio control includes monitoring of exhaust gas oxygen concentration by an exhaust sensor(s) and adjusting fuel and/or charge air parameters to meet a target air-fuel ratio. However, such feedback control may overlook cylinder-to-cylinder variation in air-fuel ratio (e.g., cylinder air-fuel ratio imbalance), which may degrade engine performance and emissions. While various approaches have been set forth for individual cylinder air-fuel control, with the aim at reducing cylinder to cylinder air-fuel ratio variation, such variation may still persist as recognized by the inventors herein. For example, issues with cylinder air-fuel ratio imbalance may include increased NOx, CO, hydrocarbon emissions, knocking, poor combustion, and decreased fuel economy.
One example approach for air-fuel imbalance monitoring is shown by Nishikiori et al. in European Patent No. 2392810. Therein, fuel is cut-off to all cylinders of an engine and an air-fuel ratio of a cylinder that combusts a mixture after fuel cut-off is monitored. An air-fuel ratio imbalance, if any, is learned and applied to the cylinder upon activation of the engine cylinders.
However, the inventors herein have recognized potential issues with such systems. As one example, Nishikiori is able to only measure an exhaust gas of the final engine cylinder fired. In this way, Nishikiori may only measure the air-fuel ratio of a single cylinder during fuel cut-off before having to initiate all the cylinders of the engine again in order to measure another cylinder air-fuel ratio. This may cause reduced drivability of the vehicle along with decreased fuel economy. As a second example, Nishikiori relies on the air-fuel sensor to accurately measure an air-fuel ratio relative to stoichiometry (e.g., the air-fuel ratio of the final combusted cylinder is compared to a measured stoichiometric air-fuel ratio). However, many issues exist with this method. For example, a geometry of the exhaust manifold and a location of an air-fuel ratio sensor, particularly for V engines, may reduce the accuracy of air-fuel ratio measurements at stoichiometry due to sensor blindness.
In one example, the issues described above may be addressed by a method for sequentially firing a cylinder group, each having a selected fuel pulse width delivered, and identifying an air-fuel ratio imbalance among each cylinder based on a deviation from a maximum lean air-fuel ratio measured during a DFSO. In this way, an air-fuel ratio imbalance may be monitored with less concern for sensor blindness.
In the view above, the inventors have recognized that a more accurate method for detecting an air-fuel imbalance may exist during DFSO (e.g., a period of low driver demand torque where the engine continues to rotate and where spark and fuel cease to be supplied to one or more engine cylinders). For example, upon measuring a maximum air-fuel ratio during a DFSO, only a selected cylinder may be fired at a time (once or multiple times during the DFSO) in order to determine an air-fuel ratio imbalance for an individual cylinder of an engine compared to an expected deviation. Each cylinder of the engine may be operated in this way during the DFSO so that all cylinder imbalances can be monitored. Further, since the combustion during the DFSO does not need to make torque to drive the vehicle, a relatively small amount of fuel may be combusted at a relatively lean overall air-fuel ratio, for example only sufficient to provide complete combustion. In this way, measurements can be provided for one cylinder at a time with minimal impact on drivability during the DFSO.
As another example, a method may be configured to monitor an air-fuel imbalance during DFSO. The air-fuel imbalance detection may initiate upon detecting a maximum lean air-fuel ratio during DFSO. A cylinder or cylinder group may be selected based one or more of a firing time and cylinder position and the cylinder or cylinder group may be fired while other cylinders remain deactivated based on the DFSO event. An air-fuel ratio of the cylinder or cylinder group may be measured and compared to an expected air-fuel ratio. If a difference between the measured air-fuel ratio and the expected air-fuel ratio is greater than a threshold, then the cylinder or cylinder group may have an air-fuel ratio imbalance. The imbalance may be learned and applied to future cylinder operations subsequent termination of the DFSO. In this way, determining an air-fuel ratio of an individual cylinder may be improved.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.